KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA

Some pfncache pages may actually be overlays on guest memory that have a
fixed HVA within the VMM. It's pointless to invalidate such cached
mappings if the overlay is moved so allow a cache to be activated directly
with the HVA to cater for such cases. A subsequent patch will make use
of this facility.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20240215152916.1158-10-paul@xen.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Paul Durrant

include/linux/kvm_host.h

@@ -148,6 +148,11 @@ static inline bool kvm_is_error_hva(unsigned long addr)
 
 #endif
 
+static inline bool kvm_is_error_gpa(gpa_t gpa)
+{
+	return gpa == INVALID_GPA;
+}
+
 #define KVM_ERR_PTR_BAD_PAGE	(ERR_PTR(-ENOENT))
 
 static inline bool is_error_page(struct page *page)
@@ -1344,6 +1349,22 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm);
  */
 int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
 
+/**
+ * kvm_gpc_activate_hva - prepare a cached kernel mapping and HPA for a given HVA.
+ *
+ * @gpc:          struct gfn_to_pfn_cache object.
+ * @hva:          userspace virtual address to map.
+ * @len:          sanity check; the range being access must fit a single page.
+ *
+ * @return:       0 for success.
+ *                -EINVAL for a mapping which would cross a page boundary.
+ *                -EFAULT for an untranslatable guest physical address.
+ *
+ * The semantics of this function are the same as those of kvm_gpc_activate(). It
+ * merely bypasses a layer of address translation.
+ */
+int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long hva, unsigned long len);
+
 /**
  * kvm_gpc_check - check validity of a gfn_to_pfn_cache.
  *
@@ -1390,6 +1411,16 @@ int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len);
  */
 void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc);
 
+static inline bool kvm_gpc_is_gpa_active(struct gfn_to_pfn_cache *gpc)
+{
+	return gpc->active && !kvm_is_error_gpa(gpc->gpa);
+}
+
+static inline bool kvm_gpc_is_hva_active(struct gfn_to_pfn_cache *gpc)
+{
+	return gpc->active && kvm_is_error_gpa(gpc->gpa);
+}
+
 void kvm_sigset_activate(struct kvm_vcpu *vcpu);
 void kvm_sigset_deactivate(struct kvm_vcpu *vcpu);
 

virt/kvm/pfncache.c

@@ -48,7 +48,14 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
 	if (!gpc->active)
 		return false;
 
-	if (gpc->generation != slots->generation || kvm_is_error_hva(gpc->uhva))
+	/*
+	 * If the page was cached from a memslot, make sure the memslots have
+	 * not been re-configured.
+	 */
+	if (!kvm_is_error_gpa(gpc->gpa) && gpc->generation != slots->generation)
+		return false;
+
+	if (kvm_is_error_hva(gpc->uhva))
 		return false;
 
 	if (offset_in_page(gpc->uhva) + len > PAGE_SIZE)
@@ -209,22 +216,27 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
 	return -EFAULT;
 }
 
-static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
+static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
 			     unsigned long len)
 {
-	struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
-	unsigned long page_offset = offset_in_page(gpa);
+	unsigned long page_offset;
 	bool unmap_old = false;
 	unsigned long old_uhva;
 	kvm_pfn_t old_pfn;
 	bool hva_change = false;
 	void *old_khva;
 	int ret;
 
+	/* Either gpa or uhva must be valid, but not both */
+	if (WARN_ON_ONCE(kvm_is_error_gpa(gpa) == kvm_is_error_hva(uhva)))
+		return -EINVAL;
+
 	/*
-	 * If must fit within a single page. The 'len' argument is
-	 * only to enforce that.
+	 * The cached acces must fit within a single page. The 'len' argument
+	 * exists only to enforce that.
 	 */
+	page_offset = kvm_is_error_gpa(gpa) ? offset_in_page(uhva) :
+					      offset_in_page(gpa);
 	if (page_offset + len > PAGE_SIZE)
 		return -EINVAL;
 
@@ -246,29 +258,39 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
 	old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
 	old_uhva = PAGE_ALIGN_DOWN(gpc->uhva);
 
-	/* Refresh the userspace HVA if necessary */
-	if (gpc->gpa != gpa || gpc->generation != slots->generation ||
-	    kvm_is_error_hva(gpc->uhva)) {
-		gfn_t gfn = gpa_to_gfn(gpa);
-
-		gpc->gpa = gpa;
-		gpc->generation = slots->generation;
-		gpc->memslot = __gfn_to_memslot(slots, gfn);
-		gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
+	if (kvm_is_error_gpa(gpa)) {
+		gpc->gpa = INVALID_GPA;
+		gpc->memslot = NULL;
+		gpc->uhva = PAGE_ALIGN_DOWN(uhva);
 
-		if (kvm_is_error_hva(gpc->uhva)) {
-			ret = -EFAULT;
-			goto out;
-		}
-
-		/*
-		 * Even if the GPA and/or the memslot generation changed, the
-		 * HVA may still be the same.
-		 */
 		if (gpc->uhva != old_uhva)
 			hva_change = true;
 	} else {
-		gpc->uhva = old_uhva;
+		struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
+
+		if (gpc->gpa != gpa || gpc->generation != slots->generation ||
+		    kvm_is_error_hva(gpc->uhva)) {
+			gfn_t gfn = gpa_to_gfn(gpa);
+
+			gpc->gpa = gpa;
+			gpc->generation = slots->generation;
+			gpc->memslot = __gfn_to_memslot(slots, gfn);
+			gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
+
+			if (kvm_is_error_hva(gpc->uhva)) {
+				ret = -EFAULT;
+				goto out;
+			}
+
+			/*
+			 * Even if the GPA and/or the memslot generation changed, the
+			 * HVA may still be the same.
+			 */
+			if (gpc->uhva != old_uhva)
+				hva_change = true;
+		} else {
+			gpc->uhva = old_uhva;
+		}
 	}
 
 	/* Note: the offset must be correct before calling hva_to_pfn_retry() */
@@ -319,7 +341,15 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
 
 int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len)
 {
-	return __kvm_gpc_refresh(gpc, gpc->gpa, len);
+	/*
+	 * If the GPA is valid then ignore the HVA, as a cache can be GPA-based
+	 * or HVA-based, not both.  For GPA-based caches, the HVA will be
+	 * recomputed during refresh if necessary.
+	 */
+	unsigned long uhva = kvm_is_error_gpa(gpc->gpa) ? gpc->uhva :
+							  KVM_HVA_ERR_BAD;
+
+	return __kvm_gpc_refresh(gpc, gpc->gpa, uhva, len);
 }
 
 void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
@@ -329,10 +359,12 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
 
 	gpc->kvm = kvm;
 	gpc->pfn = KVM_PFN_ERR_FAULT;
+	gpc->gpa = INVALID_GPA;
 	gpc->uhva = KVM_HVA_ERR_BAD;
 }
 
-int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
+static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
+			      unsigned long len)
 {
 	struct kvm *kvm = gpc->kvm;
 
@@ -353,7 +385,17 @@ int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
 		gpc->active = true;
 		write_unlock_irq(&gpc->lock);
 	}
-	return __kvm_gpc_refresh(gpc, gpa, len);
+	return __kvm_gpc_refresh(gpc, gpa, uhva, len);
+}
+
+int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
+{
+	return __kvm_gpc_activate(gpc, gpa, KVM_HVA_ERR_BAD, len);
+}
+
+int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long uhva, unsigned long len)
+{
+	return __kvm_gpc_activate(gpc, INVALID_GPA, uhva, len);
 }
 
 void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)